In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities, when compared with metals.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which incorporates one or more diverse kinds of filler materials such as particulates, fibers, rods, and the like.
There are several known limitations or difficulties in substituting ceramics for metals, such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application and costs. Several copending patent applications and patents assigned to the same owner as this application (hereinafter referred to as Commonly Owned Patents and Patent Applications), overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. The method is disclosed generically in commonly owned U.S. Pat. No. 4,713,360, which issued on Dec. 15, 1987 from copending U.S. application Ser. No. 818,943, filed Jan. 15, 1986, which was a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, and now abandoned which was a continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985and now abandoned which was a continuation-in-part of U.S. application Ser. No. 591,392, filed Mar. 16, 1984, and now abandoned all in the names of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods for Making the Same". This Patent discloses the method of producing self-supporting ceramic bodies grown as the oxidation reaction product from a parent precursor metal Molten metal is reacted with a vapor-phase oxidant to form an oxidation reaction product, and the metal migrates through the oxidation product toward the oxidant thereby continuously developing a ceramic polycrystalline body which can be produced having an interconnected metallic component. The process may be enhanced by the use of an alloyed dopant, such as is used in the case of oxidizing aluminum doped with magnesium and silicon for oxidation reaction in air to form alpha-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the precursor metal, as described in commonly owned U.S. patent application Ser. No. 220,935, filed June 23, 1988, which issued on Aug. 1, 1989, as U.S. Pat. No. 4,853,352, that was a continuation of Ser. No. 822,999, filed Jan. 27, 1986, and now abandoned which was a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, and now abandoned which was a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, and now abandoned which was a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, and now abandoned all in the names of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials".
This oxidation phenomenon was utilized in producing ceramic composite bodies as described in commonly owned U.S Pat. No. 4,851,375, which issued on July 25, 1989, from U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, which was a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, and now abandoned both in the names of Marc S. Newkirk et al and entitled "Composite Ceramic Articles and Methods of Making Same". These applications and Patent disclose novel methods for producing a self-supporting ceramic composite by growing an oxidation reaction product from a precursor metal into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration.
A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in the commonly owned and copending U.S. patent application Ser. No. 338,471, filed on Apr. 14, 1989, which is a continuation of U.S. patent application Ser. No. 861,025, filed on May 8, 1986 and now abandoned in the names of Marc S. Newkirk, et al and entitled, "Shaped Ceramic Composites and Methods of Making the Same". In accordance with the method in these U.S. patent applications, the developing oxidation reaction product infiltrates a permeable preform in the direction towards a defined surface boundary. It was discovered that high fidelity is more readily achieved by providing the preform with a barrier means, as disclosed in commonly owned U.S. Pat. No. 4,923,832, which issued on May 8, 1990, from U.S. patent application Ser. No. 861,024, filed on May 8, 1986 in the names of Marc S. Newkirk, et al and entitled, "Method of Making Shaped Ceramic Composites with the Use of a Barrier". This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a precursor metal to a barrier means spaced from the metal for establishing a boundary or surface. Ceramic composites having a cavity with an interior geometry inversely replicating the shape of a positive mold or pattern is disclosed in commonly owned U.S. Pat. No. 4,828,785, which issued on May 9, 1989, from U.S. patent application Serial No. 823,542, filed on Jan. 27, 1986, and in U.S. Pat. No. 4,859,640, which issued on Aug. 22, 1989, from U.S. patent application Ser. No. 896,157, filed on Aug. 13, 1986.
The aforementioned Commonly Owned Patent Applications and Patents disclose methods for producing ceramic articles which overcome some of the traditional limitations or difficulties in producing ceramic articles as substitutes for metals in end-use applications.
Common to each of these Commonly Owned Patent Applications and Patents is the disclosure of embodiments of a ceramic body comprising an oxidation reaction product interconnected in one or more dimensions (usually in three dimensions) and one or more metallic constituents or components. The volume of metal, which typically includes non-oxidized constituents of the parent metal and/or metal reduced from an oxidant or filler, depends on such factors as the temperature at which the oxidation reaction product is formed, the length of time at which the oxidation reaction is allowed to proceed, the composition of the parent metal, the presence of dopant materials, the presence of reduced constituents of any oxidant or filler materials, etc. Some of the metallic components are isolated or enclosed, but also a substantial volume percent of metal will be interconnected and accessible, or rendered accessible, from an external surface of the ceramic body. It has been observed for these ceramic bodies that this metal-containing component or constituent (both isolated and interconnected) can range from about 1 to about 40 percent by volume, and sometimes higher. The metallic component can impart certain favorable properties to, or improve the performance of, the ceramic articles in many product applications. For example, the presence of metal in the ceramic structure may have a substantial benefit with respect to imparting fracture toughness, thermal conductivity, resilience or electrical conductivity to the ceramic body.
The present invention discloses a method for tailoring the constituency of the metallic component (both isolated and interconnected) of such ceramics during formation of the ceramic body to impart one or more desirable characteristics to the resulting ceramic product. Thus, product design for the ceramic body is advantageously achieved by incorporating the desired metallic component in situ, rather than from an extrinsic source or by post-forming.
The entire disclosures of all of the foregoing Commonly Owned Patent Applications and Patents are expressly incorporated herein by reference.